Billionaire environmentalist Tom Steyer has threatened to launch a political assault on Republicans as well as Louisiana Sen. Mary Landrieu, a Democrat friendly to the oil and gas industry who is gearing up for a tough re-election campaign.

Steyer, a Democrat worth about $1.5 billion, leads a political organization called NextGen Climate Action. With Steyer’s funding, the group has launched a media campaign against the Keystone XL pipeline. Soon, that campaign may narrow its focus on Landrieu to criticize her for supporting the pipeline.

Steyer’s super PAC, in its own words online, is dedicated to “supporting candidates … that will take bold action on climate change -- and to exposing those who deny reality and cater to special interests.”

Earlier this month, NextGen Climate Action asked online for users to choose a candidate for the group to target with a TV ad. That list included Sen. Marco Rubio (R-Fla.), up for re-election in 2016, and three Senate hopefuls: Rep. Shelley Moore Capito (R-W.Va.), former South Dakota Republican Gov. Mike Rounds and Rep. Paul Broun (R-Ga.). Landrieu was the only Democrat on the list.

Landrieu has served three terms as senator and has received $1.23 million in campaign donations from the oil and gas industry since 1989, according to OpenSecrets.org. On Thursday, she was named the new chair of the Senate Energy and Natural Resources committee.

If Steyer attacks Landrieu, he may weaken Democratic control of the Senate. But he also may strengthen Landrieu’s campaign if voters in Louisiana, a state dependent on oil and natural gas, begin liking her more for enduring criticism from the environmental Left.

Steyer made his fortune as a hedge fund manager at Farallon Capital, in part on investments related to fossil fuels. He plans to spend as much as $100 million on the 2014 elections.